[samtrion]

Problem

Latest mutation run (40488172984): 193 mutants created, 36 mutants got status CompileError, 37 mutants got status Ignored, score 78.33 %. According to the report, most of the compile errors (31 of 36) are "Linq method mutation (Append() to Prepend())" on StringBuilder.Append chains in XUnitLogger.cs (not independently re-verified; the report JSON was not available when preparing this PR). src/ has no System.Linq usage.

Root cause

The Linq mutator matches by method name only, so StringBuilder.Append() is treated as Enumerable.Append() and rewritten to Prepend(), which does not compile. The mutator is Standard level, so it is active under "mutation-level": "Advanced".

Change

Add "ignore-mutations": ["linq"] to stryker-config.json. Revisit this if System.Linq is ever used in src/. ignore-methods for Append was deliberately not used because it would also drop real mutants on those lines.

Before / After

metric	before	after
score	77.21 %	78.46 %
mutants created	229	223
compile errors	n/a	52
NoCoverage	5	3
ignored	41	41
tested (excl. NoCoverage)	131	127
killed	105	102
survived	26	25

Run: https://github.com/dailydevops/fluentvalue/actions/runs/40804569084 (success).

Result: 3 non-killed mutants removed (2 NoCoverage, 1 survivor), as predicted; killed dropped by 3 because equivalent mutants in the removed code disappeared. The score gain is modest (+1.25 points). The remaining 28 non-killed mutants (3 NoCoverage, 25 survived) are not addressed here.

Risk / what this does not change

Score should be unchanged: neither CompileError nor Ignored mutants count toward it. Success is measured by the CompileError and Ignored counts, not the total. Saves failed compile attempts, since ignored mutants are not compiled. No source or test changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b5aad10) to head (ed0d3e4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #612      +/-   ##
===========================================
+ Coverage   98.54%   100.00%   +1.45%     
===========================================
  Files          22        22              
  Lines         274       261      -13     
  Branches       43        38       -5     
===========================================
- Hits          270       261       -9     
+ Misses          1         0       -1     
+ Partials        3         0       -3     

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